Page 287 - Gas Adsorption Equilibria

5. Oscillometry 273

for pressures (p > 5 MPa). The later effect is due to the swelling of the
polymer/gas system increasing in this way “the dead space” for the sorptive
gas phase. The reduced masses gained by oscillometric measurements
increase monotonously with increasing gas pressure (p). So do the data
for the specific volume of the polymer/gas system which
can be correlated linearly by

Here is the volume of the PMMA at T, k(T) is the
temperature dependent Henry coefficient of the swelling isotherm, is the
molar volume of the sorptive gas at (p, T) and is the
standard molar volume of an ideal gas at p = 1 atm, T = 273,15 K.

The data for the gas mass sorbed also increase with increasing pressure
showing deviation from purely linear behavior. Fitting of these data is
possible by using the isotherm

which is a superposition of a linear term reflecting bulk sorption of gas in the
polymer, and a Langmuirian term describing adsorption of a limited amount
of gas on the surface of “micro cavities” formed by the chain like molecules
of the polymer [5.33]. Equilibration times of the sorption process lasted from
6 hours at low gas pressures up to 2 days at high gas pressures
Relative uncertainties of all data are about 5 %. A comparison
of the swelling volume data with some data from the literature is given in Fig.
5.20. Agreement within experimental uncertainties – as far as those can be
guessed for literature data – seems to be reasonable.